This invention relates to a flow modifier for aqueous systems which is also an effective adsorbate onto solids from aqueous systems. The modifier is a copolymer of acrylamide and an N-substituted acrylamide. The N-substituent comprises a hydrophobic group of an effective size. Aqueous solutions of such an acrylamide copolymer have higher viscosities than aqueous solutions of a polyacrylamide of similar molecular weight without hydrophobic groups, when measured at concentrations such as 0.1% or more. Multiphase compositions, comprising a continuous aqueous phase and at least one other phase, containing the acrylamide copolymer flow modifier have a surprising combination of desirable properties, particularly rheological properties, in a number of uses. Typical of these uses are paint and printing ink modifications, emulsion stabilization, and pigment dispersion.
High molecular weight polyacrylamide and partially hydrolyzed derivatives thereof have long been known as thickeners for water and in various other uses as reported in "Handbook of Water Soluble Gums and Resins", R. L. Davidson, Ed., McGraw Hill 1980, Chapter 16 by Volk and Friedrich. Processes for preparing polyacrylamide are old in the art; Minsk et al., U.S. Pat. No. 2,486,191, teach solution polymerization in aqueous alcohol solution, Monagle et al., U.S. Pat. Nos. 3,336,269, 3,336,270 and 3,509,113, teach a precipitation process employing tertiary butanol, and optionally acetone, in water as the polymerization medium.
A large hydrocarbyl substituent on the amide nitrogen of the required comonomer is believed to be responsible for leading to many of the unusual and unexpected properties of the acrylamide copolymers of the instant invention and of their aqueous solutions. Among these properties is the surprising increase in viscosity of an aqueous solution as the concentration of the polymer is increased, as compared to polyacrylamide.
Similarly, unusual behavior is found in the rheological properties of emulsions, suspensions and other multiphase systems, such as those containing both liquid and solid phases (mixed emulsions and suspensions), comprising this acrylamide copolymer in aqueous solution or at an aqueous interface. Further novel effects are noted when systems containing the acrylamide copolymer of the instant invention are dried and ease of bonding to the exposed surface is determined. It is seen that bonding is poor, especially the bonding between two coated surfaces (i.e., so-called "blocking") and it is concluded that the acrylamide copolymer is a surprisingly adhesion inhibiting adsorbate. The thickeners or flow modifiers of the invention are exceptionally versatile in their ability to impart special rheological properties to aqueous solutions and to multiphase solutions comprising an aqueous continuous phase. They are relatively efficient viscosity improvers at comparatively low molecular weights and at the same time provide improved rheological properties to aqueous solutions and systems subjected to high or low shear rate. An example of these improvements is found in latex paints containing the modifiers of this invention formulated so as to provide, in a single paint formulation, superior flow and leveling, desired viscosity values under both low shear and high shear conditions and applied coatings having high gloss, and resistance to blocking.
It is believed that a number of the unusual properties of the acrylamide copolymer, for example, latex stabilization and block improvement, are aided by adsorption of the polymer from the aqueous phase onto solid surfaces, such as latex particle surfaces and pigment surfaces, which may be present. The adsorption results in the presence of thickener molecules at the surface of films, formed upon drying a latex or other solid-containing formulations utilizing the acrylamide copolymer, which presence leads to unexpected surface properties.
Water soluble thickeners are notoriously difficult to handle in industrial processes because, when furnished as solids they take inordinately long times to dissolve, or when furnished as concentrated solutions they are difficult to dilute uniformly so as to avoid the production of local areas of high concentration, often known as "fish-eyes" because of their appearance. Both in simple aqueous solution and, more particularly, in formulations comprising other phases, extended agitation and aging periods are required before an equilibrium viscosity is attained. This requires formulators to guess or develop rule of thumb methods for estimating the final equilibrium rheological properties of the formulations or to inventory large amounts of product and reprocess it if the final rheological properties do not meet an established specification. One of the advantages of the acrylamide copolymers of this invention is the rapidity with which equilibrium rheological properties are attained. This rapid equilibration may be due to the high viscosity of the aqueous copolymer solutions, at concentrations above about 0.1%, for such relatively small copolymer molecules. Thus, one may regard the aqueous solutions as having a viscosity to be expected of high molecular weight polyacrylamides while, at the same time, insofar as diffusional characteristics, such as the rate of concentration equilibration, are concerned, the acrylamide copolymer behaves as though the molecular weight is comparatively low. This is both a surprising property and one of great importance in the manufacture of multiphase systems.
The rapid equilibration, in the distribution in solution, of the acrylamide copolymers of this invention may be considered to be associated with the unusual rheological properties of systems thickened thereby. Without being bound by the theories expressed or implied, the following remarks are offered as an aid in the teaching of this invention. The introduction of these thickener polymers into a solution is thought to promote a specific structure in the solution phase or an interaction among phases resulting in a structure. If the structure is resistant to change throughout a broad range of shear rates, then its rheology profile is Newtonian, that is, its apparent viscosity is relatively independent of shear rate. If the structure is not shear resistant, but degrades under shear and does not reform as fast as the degradation, then its rheological profile will be less Newtonian and more pseudoplastic.
A water soluble high molecular weight polymer added to an aqueous phase readily gives an increased viscosity as measured at a given low shear rate. However, as the shear rate is increased, it is commonly observed that the viscosity diminishes, that is, the solution appears to have pseudoplastic rheology. This may be due to the original structure not being maintained because the high molecular weight polymer molecules do not permit rapid equilibration of their distribution in the solution. Because of the unexpectedly high rate of equilibration, the thickeners of this invention exhibit a Newtonian-proximate rheology even at high shear rates.
The nonionic acrylamide copolymers of this invention have outstanding tolerance for electrolytes in the sense that the rheological properties of their aqueous solutions are comparatively insensitive to electrolytes as well as to changes in pH. The lack of sensitivity to pH changes is, in part, due to the stability to hydrolysis of the linkage between the hydrophobic group and the polymer backbone. Most polymer latex particles and other solids in industrial use being nonionic or anionic, the anionic acrylamide copolymers of this invention are of widespread applicability even if less tolerant of electrolytes. However, cationic acrylamide copolymers would not be so useful for they would lead to floccolation and gellation of such systems as well as being electrolyte and pH sensitive. The viscosity of aqueous solutions thickened by acrylamide copolymers of this invention is less sensitive to shear than solutions of the same viscosity thickened by prior art materials.
The acrylamide copolymers can be prepared with hydrophobe-containing chain transfer agents and, optionally, hydrophobe-containing initiators.
Formulated with a wide variety of aqueous systems comprising small particles, such as latexes, these acrylamide copolymers afford multiphase systems of great commercial interest such as paints, printing inks and other systems comprising a thickened aqueous phase. An example of such materials are gloss paints which are found to have an excellent balance of low shear and high shear viscosity, exceptional flow, leveling and film build and which produce resulting paint films having high gloss, block resistance, and, when compared to formulations comprising conventional nonionic thickeners such as hydroxyethyl-cellulose or anionic thickeners, have better alkali and abrasion resistance and resistance to degradation such as by shearing or by enzymes. In general, the acrylamide copolymers of the instant invention are also effective sedimentation stabilizers, surfactants, and dispersants.